[[ports]]
== NIFs and port drivers

Erlang.mk can not only build Erlang projects, but also the C code
that some projects come with, like NIFs and port drivers.

There are two ways to build the C code: using a custom Makefile,
or making Erlang.mk do it directly. The C code will be built
as needed when you run `make`.

// @todo something for easier bootstrapping

=== C source code location and Erlang environment

The C source code should be located in the '$(C_SRC_DIR)' directory.
It defaults to 'c_src/'. Should you need to modify it, all you
need to do is to set the variable in your Makefile before including
Erlang.mk:

[source,make]
C_SRC_DIR = $(CURDIR)/my_nif_source

When this directory exists, Erlang.mk will automatically create a
file named '$(C_SRC_ENV)'. This file defaults to '$(C_SRC_DIR)/env.mk'.
This can also be changed:

[source,make]
C_SRC_ENV = $(C_SRC_DIR)/erlang_env.mk

It contains a few variable definitions for the environment used for the build:

`ERTS_INCLUDE_DIR`::
	Path to the ERTS include files ('erl_driver.h', 'erl_nif.h' and more).
`ERL_INTERFACE_INCLUDE_DIR`::
	Path to the Erl_Interface include files ('ei.h' and related).
`ERL_INTERFACE_LIB_DIR`::
	Path to the Erl_Interface static libraries.

=== Using a custom Makefile

Erlang.mk will automatically run `make` if it detects a Makefile
in '$(C_SRC_DIR)/Makefile'.

The Makefile should have at least two targets: a default target
(which can be anything, for example `all`) which is invoked when
building the C code, and a `clean` target invoked when cleaning
it.

You can include the 'env.mk' file to benefit from the Erlang
environment detection:

[source,make]
include env.mk

=== Using Erlang.mk directly

You don't need to write a Makefile to build C source code, however.
Erlang.mk comes with rules to build both shared libraries and
executables, using the source files it finds in '$(C_SRC_DIR)'.

By default, Erlang.mk will create a shared library. To change
this and create an executable instead, put this in your Makefile
before including Erlang.mk:

[source,make]
C_SRC_TYPE = executable

The generated file name varies depending on the type of project
you have (shared library or executable) and on the platform you
build the project on.

For shared libraries, the generated file name will be
'$(C_SRC_OUTPUT)$(C_SRC_SHARED_EXTENSION)', with the default
being '$(CURDIR)/priv/$(PROJECT)' followed by the extension:
`.dll` on Windows, `.so` everywhere else.

For executables, the generated file name is
'$(C_SRC_OUTPUT)$(C_SRC_EXECUTABLE_EXTENSION)', with the same
default except for the extension: `.exe` on Windows, and otherwise
nothing.

Erlang.mk sets appropriate compile and linker flags by default.
These flags vary depending on the platform, and can of course
be overridden.

`CC`::
	The compiler to be used.
`CFLAGS`::
	C compiler flags.
`CXXFLAGS`::
	C++ compiler flags.
`LDFLAGS`::
	Linker flags.
`LDLIBS`::
	Libraries to link against.

The source files are automatically gathered from the contents
of '$(C_SRC_DIR)'. Erlang.mk looks for '.c', '.C', '.cc' and '.cpp'
source files. You can define the variable `SOURCES` to manually
list the files to compile.

=== Propagating compile and linker flags to sub-Makefiles

In some cases it might be necessary to propagate the flags
you just defined to the sub-Makefiles of your local project.
You generally can't just export those as this could impact
the building of dependencies.

Makefiles allow you to export variables for specific targets.
When doing this, the variables will be exported only when
this target runs, and not for other targets. It is therefore
possible to export them when building the C code without
impacting other build steps.

By adding this to your Makefile all five variables will be
made available to sub-Makefiles when building C code:

[source,make]
----
app-c_src: export CC +=
app-c_src: export CFLAGS +=
app-c_src: export CPPFLAGS +=
app-c_src: export LDFLAGS +=
app-c_src: export LDLIBS +=
----

Appending an empty string to the existing value is necessary
because Makefiles expect an assignment for target-specific
exports. Alternatively you can set a new value:

[source,make]
----
app-c_src: export CFLAGS = -O3
----